1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and more particularly to an apparatus and method for signal transmission and reception between a Mobile Station (MS) and a Base Station (BS) when a change occurs in downlink channel information while the MS is in a sleep mode.
2. Description of the Related Art
In a 4th generation (4G) communication system, which is the next generation communication system, research has been pursued to provide users with services having various qualities of service (QoSs) at a high transmission speed. Recently, in the 4G communication system, research has been actively pursued to support high speed services while ensuring mobility and QoS for Broadband Wireless Access (BWA) communication systems such as a wireless local area network (LAN) and a metropolitan area network (MAN) system. A representative communication system designed in order to achieve such goals as described above includes an IEEE (Institute of Electrical and Electronics Engineers) 802.16e communication system.
The IEEE 802.16e communication system utilizes an Orthogonal Frequency Division Multiplexing (OFDM) scheme and an Orthogonal Frequency Division Multiple Access (OFDMA) scheme in order to support a broadband transmission network for a physical channel of the wireless MAN system.
FIG. 1 is a block diagram schematically illustrating a conventional IEEE 802.16e communication system. Referring to FIG. 1, the IEEE 802.16e communication system has a multi-cell structure including a cell 100 and a cell 150. Also, the IEEE 802.16e communication system includes a BS 110 controlling the cell 100, a BS 140 controlling the cell 150, and a plurality of MSs 111, 113, 130, 151, and 153. The transmission and reception of signals between the BSs 110 and 140 and the MSs 111, 113, 130, 151, and 153 is accomplished using an OFDM/OFDMA scheme. Herein, the MS 130 is located in a boundary area, i.e., handover area, between the cell 100 and the cell 150. Accordingly, when the MS 130 moves into the cell 150 controlled by the BS 140 while transmitting and receiving with the BS 110, the serving BS for the MS 130 changes from the BS 110 to the BS 140.
In the IEEE 802.16e communication system, the power consumption of the MS plays an important part in the performance of the entire system. Therefore, a sleep mode operation and an awake mode operation corresponding to the sleep mode operation have been proposed for the BS and the MS in order to minimize the power consumption of the MS. Further, in order to cope with a channel state change between the MS and the BS, the MS periodically performs ranging for adjusting the timing offset, the frequency offset, and the transmit power between the BS and the MS.
Hereinafter, an operation for downlink burst profile allocation in a typical IEEE 802.16a communication system will be described.
First, when the MS is powered on, the MS monitors all frequency bands set in advance in the MS and detects a pilot signal having a largest intensity, i.e., a largest Carrier to Interference and Noise Ratio (CINR). Further, the MS determines a BS transmitting the pilot signal having the largest CINR as the serving BS, which is a BS to which the MS currently belongs. Then, the MS receives a preamble of a downlink frame transmitted from the serving BS and acquires system synchronization between the MS and the serving BS.
When the MS synchronizes to the serving BS, the serving BS transmits a DL(Downlink)_MAP message and a UL(Uplink)_MAP message to the MS. The DL_MAP message has a message format as shown in Table 1 below.
TABLE 1SyntaxSizeNotesDL_MAP_Message_Format ( ) { Management Message Type=28 bits PHY Synchronization FieldVariableSee Appropriate PHYspecification     DCD Count 8 bits    Base Station ID48 bits Number of DL_MAP Element n16 bitsBegin PHY Specification section{See Applicable PHYsection   for (i=1; i<=n; i++)For each DL_MAPelement 1 to nDL_MAP Information Element( )VariableSee corresponding PHYspecification   If !(byte boundary) { 4 bitsPadding to reach byteboundary   Padding Nibble      }     }   } }
As shown in Table 1, the DL_MAP message contains a plurality of Information Elements (IEs), such as ‘Management Message Type’ representing a type of a message being currently transmitted, ‘PHY synchronization’ set correspondingly to the modulation scheme and demodulation scheme applied to a physical (PHY) channel for acquisition of synchronization, ‘DCD count’ representing a count corresponding to changes in a configuration of a Downlink Channel Descriptor (DCD) message including a downlink burst profile, ‘Base Station ID’ representing a BS identifier, and ‘Number of DL_MAP Elements n’ representing the number of the elements following the ‘Base Station ID’. The DL_MAP message in Table 1 contains n number of DL_MAP IEs, each of which includes a Downlink Interval Usage Code (DIUC) that has a value mapped to a downlink burst profile included in the DCD message. That is, the MS can detect information about the coding scheme (Forward Error Correction (FEC) code type) and modulation scheme applied to downlink bursts included in the downlink frame by extracting the DIUC value from the DL_MAP message. Accordingly, the MS can receive the data (data frame) in the downlink burst while identifying the downlink bursts in the downlink frame.
When movement of the MS occurs or a change in the surrounding channel conditions of the MS causes change in the CINR value of the pilot signal received by the MS from the serving BS, it is necessary to also change the DIUC value applied to the data to be transmitted by the MS, correspondingly to the change in the CINR value of the pilot signal.
Further, when it is necessary for the BS to change the downlink burst profile, the BS changes the burst profile and then transmits a DCD message including the information about the change to the MS. Then, by receiving the DCD message, the MS can recognize the change in the downlink burst profile from the DCD message.
However, when the downlink burst profile changes, i.e., the DCD message changes, while the MS is in a sleep mode, the MS cannot recognize the change of the DCD message in realtime because the MS is in the sleep mode.
FIG. 2 schematically illustrates an operation of an MS when the DCD message changes while the MS is in a sleep mode in a conventional IEEE 802.16e communication system. In FIG. 2, the MS and the BS set up protocols about transmissible and receivable modulation schemes and coding schemes in advance for signal transmission and reception between them. The setup of protocols about the modulation schemes and coding schemes is achieved through transmission and reception of the downlink burst profile, i.e. transmission and reception of the DCD message.
Further, it is necessary to prepare protocols about the modulation schemes and coding schemes between the BS and the MS in order to enable the MS to normally restart data transmission/reception after awakening from the sleep mode.
However, because the MS does not receive any signal from the BS at all during the sleep interval in which the MS stays in the sleep mode, the MS cannot recognize any change in the downlink burst profile, i.e., DIUC set, made by the BS in the sleep interval. When the DIUCs used by the MS and the BS do not coincide due to the sleep mode operation of the MS, it is impossible to transmit and receive data between the MS and the BS.
Hereinafter, different scenarios in which the DIUCs used by the MS and the BS no longer coincide due to the sleep mode operation of the MS will be discussed.
The first case corresponds to when the DCD message changes while the MS performs the sleep mode operation, i.e., while the MS stays in the sleep interval.
In the IEEE 802.16e communication system, the MS detects the DCD count included in the currently received DL_MAP message and compares the detected value with the DCD count value currently stored in the MS itself. If the DCD count value included in the DL_MAP message currently received by the MS and the DCD count value currently stored in the MS are different, the MS recognizes the change of the DCD message. That is, because the DCD count values are different, the version numbers of the downlink burst profiles are different. Therefore, the MS can recognize the version number of the downlink burst profile using the DCD count value.
However, in the current IEEE 802.16e communication system, it is impossible to notify the change in the downlink burst profile to an MS that has awakened from the sleep mode. Therefore, if the BS transmits downlink data to the MS using the downlink burst profile of the BS itself, without recognizing that the downlink burst profile of the BS is different from the downlink burst profile stored in the MS, the MS cannot normally receive the downlink data.
The scenario above will be described in more detail hereinafter with reference to FIG. 2.
First, however, it is noted that FIG. 2 is based on an assumption that the parameter denoting the DCD count value managed by the BS 200 is N, the parameter denoting the DCD count value managed by the MS 250 is M, and the two parameters N and M have an initial value of ‘0’. When the BS 200 detects that it is necessary to change the downlink burst profile while the MS 250 stays in the sleep mode, i.e. in the sleep interval, the BS sets the DCD count value N, which is managed by the BS 200, to be ‘1’ (N=1) in step 211 and transmits the changed DCD message in step 213. Although the BS 200 transmitted the changed DCD message, the MS 250 in the sleep interval cannot recognize the change of the DCD message. Therefore, the MS 250 maintains the value ‘0’ (M=0) of the parameter denoting the DCD count value, which is managed by the MS 250 itself, in step 215.
When the sleep interval terminates, the MS 250 receives the DL_MAP message from the BS 200 in the listening interval in step 217. In the DL_MAP message, the DCD count value, more specifically, the value of the parameter N representing the DCD count value managed by the BS 200, is set as ‘1’. Therefore, the MS 250 recognizes from the DCD count value that it is necessary to receive a new DCD message from the BS 200.
After the listening interval terminates, the MS 200 mode-transits into the awake mode and waits for the DCD message in step 219. When the BS 200 detects occurrence of a data targeting the MS 250 while the MS 250 stays in the awake mode, the BS 200 transmits to the MS 250 a TRF_IND message indicating that there is a data to be transmitted targeting the MS 250, i.e. a TRF_IND message in which a bit representing the MS 250 in a sleep identifier bitmap is marked by a positive value, i.e., ‘1’, in step 221.
After transmitting the TRF_IND message, the BS 200 transmits data to the MS 250 in step 223. However, as described above, although the BS 200 transmits the data using the newly changed downlink burst profile, the MS 250 still uses the downlink burst profile before the change. As a result, the DIUC value applied to the data transmitted from the BS 200 to the MS 250 is different from the DIUC value stored in the MS 250, and the MS 250 cannot normally demodulate the data transmitted from the BS 200 in step 225.
As described above, in the first scenario, the DIUCs used by the MS and the BS become different, i.e., no longer coincide, due to change of the DCD message while the MS performs the sleep mode operation, i.e., while the MS stays in the sleep interval.
However, as will be described below, in the second scenario, the DIUCs used by the MS and the BS become different due to the sleep mode operation of the MS because the MS itself changes the DIUC value to be proper for the MS while the MS performs the sleep mode operation, i.e., while the MS stays in the sleep interval.
More specifically, when the MS moves while it is in the sleep interval or when the surrounding channel condition of the MS changes to make the CINR value of the pilot channel signal from the serving BS become different from that before the MS mode-transits into the sleep mode, the MS changes the DIUC value to be most proper for the MS itself. For example, when the CINR value of the pilot channel signal measured, while the MS stays in the sleep interval, becomes smaller than the CINR value of the pilot channel signal measured before the MS comes into the sleep interval, if the MS transmits data to the BS using the existing DIUC value without update, it is highly possible that the data transmitted by the MS has an error.
As described above, in the current IEEE 802.16e communication system, if the DCD message changes while the MS is in the sleep mode, it is impossible to normally perform data transmission and reception, thereby causing data loss. The data loss may degrade the general performance of the IEEE 802.16e communication system targeting a high speed data transmission. Therefore, a need exists for a solution for data transmission and reception reflecting the DCD message change in realtime.